1. Field of the Invention
Exemplary embodiments of the present invention relate to a copier, printer, facsimile machine, or similar image forming apparatus, and more particularly, to an image forming apparatus for transferring a toner image formed on a toner image carrying belt (e.g., an intermediate transfer belt) onto a sheet-like recording medium directly or via another toner image carrying member (e.g., a sheet conveyance member) to obtain a complete full-color image.
2. Discussion of the Related Art
In recent years, full-color image forming apparatuses for electrophotographic printing generally perform either a direct image transfer operation or an indirect image transfer operation. In the indirect transfer operation, respective single-color toner images formed on each image carrier that contacts an intermediate transfer belt are sequentially transferred onto the intermediate transfer belt by an electric field supplied by a transfer bias unit to form a composite toner image, in an operation that is referred to as primary transfer. Then, the composite toner image retained by the intermediate transfer belt is transferred onto a recording medium conveyed along the intermediate transfer belt, in an operation referred to as secondary transfer. Through the primary and secondary transfers, ultimately a full-color toner image is formed on the recording medium.
The image carrier used for the indirect transfer operation can be a drum-shaped photoconductor, a belt-shaped photoconductor, etc. By contrast, in the direct transfer operation, respective single-color toner images formed on each image carrier can be transferred onto a recording medium directly.
In an effort to further improve image quality, various techniques have been proposed to enhance toner image transferability.
One approach, for example, discloses an ultrasonic vibration generator that generates and applies ultrasonic vibrations to a transfer position of the toner image from a toner image carrying belt or an intermediate transfer belt onto a recording medium. This approach proposes a configuration that includes a corona generating unit disposed facing a surface of the toner image carrying belt extended between two extension rollers. In this configuration, a toner image is transferred onto the recording medium when the recording medium passes between the corona generating unit and the extended surface portion of the toner image carrying belt. The ultrasonic vibration generator includes a vibration imparting part that contacts an inner side of the extended surface portion of the toner image carrying belt.
This configuration further employs a non-contact transfer system. However, such a configuration uses an electrostatic force to closely contact the toner image carrying belt and a recording medium, is susceptible to poor transferability due to insufficient contact.
Another approach discloses a transfer roller and an ultrasonic vibration generator having a vibration imparting part. The transfer roller is charged with a transfer bias having a polarity opposite that of toner, and is disposed facing one of a set of an extension rollers extending the toner image carrying belt, with the toner image carrying belt interposed between the rollers. The vibration imparting part of the ultrasonic vibration generator is disposed facing the extension roller that contacts the transfer roller via the toner image carrying belt. More particularly, the extension roller set contacting the transfer roller via the toner image carrying belt includes multiple individual rollers arranged along its axial direction. The vibration imparting members contact the inner surface of the toner image carrying belt between the multiple rollers of the extension roller set.
In the configuration according to this approach, the transfer roller is pressed against one of the multiple rollers of the extension roller set via the toner image carrying belt to form a transfer nip. As a recording medium passes through the transfer nip, stable close contact between the recording medium and the toner image carrying belt can be achieved, resulting in a stable transfer process.
In this configuration, however, multiple narrow-width rollers are arranged along an axial direction of the secondary transfer roller. Consequently, the pressure exerted by each of the rollers on the belt differs depending on whether or not the roller contacts the toner image carrying belt along the axial direction of the transfer nip, which tends to cause uneven transfer and wrinkle the recording medium as the recording medium passes therethrough. Further, it is difficult for the toner image carrying belt to vibrate easily at the transfer nip where the two rollers meet and contact each other. The upshot is that toner image transferability cannot be enhanced by applying an ultrasonic vibration to such a configuration.
It is possible to have an arrangement in which the image forming apparatus includes an ultrasonic vibration generator for applying an ultrasonic vibration to a toner image carrying belt at a different position from where the two rollers contact each other as described above. In such a configuration, a transfer-roller opposed roller and a secondary transfer roller are used. The transfer-roller opposed roller serves as an extension roller to which a transfer bias having the same polarity as that of the toner, is applied from a transfer power source. Together, the transfer-roller opposite roller and the secondary transfer roller form a contact area, which is also referred to as a secondary image transfer nip. In the secondary image transfer nip, a toner image formed on the intermediate transfer belt is transferred onto a transfer sheet serving as a recording medium. Of note is that the ultrasonic vibration generating unit includes a vibration imparting part disposed to contact the inner side of the extended surface portion of the intermediate transfer belt in an area that is located in between adjacent intermediate transfer belt rollers.
However, even this configuration was found not to improve image transferability significantly compared to a configuration without the ultrasonic vibration generator. Nor was image transferability improved when the target object onto which the toner image is transmitted is not the recording medium but another toner image carrying belt.